A person can protect/conceal their telephone number and other contact information, for example, by requesting that their contact information not be listed or otherwise made available by third parties with whom they share the contact information. One example involves a person requesting that their telephone number be treated as an unlisted number by a telephone directory service. Being unlisted prevents all people from accessing, through the directory service at least, the person's contact information. Being unlisted involves a trade-off or cost, therefore, of preventing desired contacts in order also to prevent undesired contacts. Further, the person who requests to have their telephone number unlisted is not provided an indication of who, if anybody, tries to access the person's contact information using the directory service.
Various services provide a person with information on who is calling the person. For example, a person can screen calls using an answering machine, defer calls using voicemail, or even find out (in certain situations) the phone number or calling area of a caller through caller ID. However, in each of these latter situations, the caller has already obtained the person's contact information and could repeatedly contact (or attempt to contact) the person.